Laser spectroscopy is currently enabling a large number of experiment for scientific investigation. The development of new spectroscopic method and apparatuses is still in progress, to always obtain better monitoring and precise quantitative measurements of the physical world.
Optical frequency combs (OFCs) led to impressive advances in the field of metrology and fundamental physics as detailed for example in J. Ye and S. T. Cundiff “Femtosecond optical frequency comb technology: principle, operation and application” (Springer, New York, 2005). In addition, OFCs have been proposed for a number of different applications, such as characterization of dispersion in optical materials, absolute length measurements, strain sensing, cavity-enhanced and Fourier Transform molecular spectroscopy.
Optical resonator-based detection methods, either realized with conventional mirror cavities or optical fibers and waveguides, have demonstrated a huge potential in spectroscopic and sensing applications such as in T. Gherman and D. Romanini “Mode-locked cavity-enhanced absorption spectroscopy” Optic Express Vol. 10, n° 19, pages 1033-1042 (2002).
In this field, a major breakthrough was represented by coherent coupling of OFCs to high finesse cavities used as sample chambers. Spectral analysis of the light transmitted by the cavity can be performed by dispersive elements to extract the absorption features over several tens of nm. Such systems exploit the intrinsic advantages of comb lasers, i.e. wide spectral coverage with a multi-wavelength coherent source and metrological-grade frequency stability, while preserving the intrinsic sensitivity of cavity-enhanced methods. Nevertheless, comb-based absorption spectrometers have rested on conventional linear cavities and used only for spectroscopy in the gas phase.
So far, there have been only a few works on absorption spectroscopy in the liquid phase. Liquid spectroscopy has a huge potential for analysis of many species in the liquid phase, e.g. in environmental pollution and industrial process monitoring as well as food safety control and biomedical analysis. Common cavity enhanced techniques, originally developed for gas spectroscopy, present well-known drawbacks when extended to liquid compounds. For instance, introduction of liquids in a high reflectivity mirror cavity directly or by means of an intracavity couvette causes a significant loss increase and additional reflections. An alternative and minimally-invasive method, is the use of total internal reflection at the interface between two media with different dielectric constants. For instance, in optical waveguide resonators, the interaction with liquid chemicals in the surrounding environment may occur if the internal evanescent field is exposed along the cavity-medium interface, as discussed in von Lerber T., Sigrist M. W. “Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing.” Appl. Opt. 41, 3567-3575 (2002). In fiber-optic resonators, this is possible by creating a side-polished region where the external cladding is removed while the total internal reflection condition is still satisfied. A change in ambient refractive index leads to a wavelength shift of the cavity modes and may increase the penetration depth of the evanescent-wave tail. On the other hand, if the interacting molecules exhibit optical absorption features in the vicinity of the resonance, the lifetime of photons within the cavity is reduced as a consequence of loss increase. The use of optical fiber evanescent-wave sensors has several advantages. They are particular suitable for in-situ, non-invasive sensing, and they can be used both in hardly-accessible and harsh environments even in remote operation. Additionally, fiber optic based resonators are cheap, compact, easy to build and do not require special care in terms of alignment, cleaning and isolation.
In the work of M. J. Thorpe et al. “Broadband cavity ringdown spectroscopy for sensitive and rapid molecular detection”, Science 311, 1595-1599 (2006) a femtosecond pulse train generated with a mode-locked laser source is coupled into a resonator cavity including a sample to be investigated. The femtosecond pulse train and the resonator cavity are tuned such that the cavity modes essentially corresponds to the comb components of the femtosecond pulse train. Due to the large number of round trips within the cavity, the interaction of the sample with light is essentially increased. Absorption profiles are measured by using the cavity ringdown technique. The comb beam is switched off through an acusto-optical modulator (AOM). The light transmitted through the cavity is spectrally resolved and detected via a CCD camera. For measuring the ring down exponential decays, a scanning mirror is used for streaking the spectrally resolved light pulses into different portions of the sensitive area of the CCD sensor.
U.S. Pat. No. 8,120,773 discloses a spectroscopic analysis of a sample which includes arranging the sample in a resonator cavity for transmitting cavity mode frequencies with a cavity mode frequency spacing, coupling pulsed source light into the resonator cavity, with the source light including source comb frequencies with a source frequency spacing, coupling pulsed transmitted light out of the resonator cavity, and spectrally resolved detecting the transmitted light with a detector device. The cavity mode frequency spacing and the source frequency spacing are detuned relative to each other, so that the transmitted light includes transmitted comb frequencies with a spacing larger than the source frequency spacing. The detecting feature includes collecting spectral distributions of the transmitted light in dependence on relative positions of the cavity mode frequencies and the source comb frequencies. The cavity mode frequencies and the source comb frequencies are varied relative to each other and different transmitted comb frequencies are individually resolved.